1. Field of the Invention
This invention relates to tunable optical components.
2. Description of the Related Art
Industry experts agree that the telecommunications industry is experiencing explosive growth and is one of today""s fastest growing economic segments. With the tremendous growth of the Internet and the increase in telecommunications traffic, many telecom companies are rapidly deploying new network topologies and transport technologies such as WDM (wavelength-division-multiplexing) and DWDM (dense-wavelength division multiplexing) to increase the capacities of their networks. With the advent of fiber optic communications networks, the deployment of all-optical networks is clearly the ultimate goal for the next generation of telecommunications networks. Critical to the successful deployment of the all-optical network is the development of cost effective tunable optical components such as tunable filters, tunable laser sources, tunable dispersion compensators (both chromatic and polarization mode) tunable add/drop multiplexers, etc.
MEMS devices are a promising new class of tunable optical components. These devices generally comprise an array of small (ca. micron sized) moving parts, which are manipulated into desired configurations, to actuate an optical member. For instance, an array of micromirrors can be manipulated to create an optical cross connect switch, a pair of parallel mirrors can be manipulated to create a tunable Fabry-Perot Interferometer, etc.
Currently, the vast majority of MEMS devices are constructed with silicon, metallic or glassy hinges, which anchor a moving, part (e.g. a micromirror) to a substrate, which typically contains a control electrode. When a command signal in the form of a voltage is applied between the electrode and the moving part, the moving part moves against the restoring force exerted by the hinge. Use of these silicon/glassy/metallic materials for hinges creates engineering hurdles, which severely limits the design space of the MEMS device. Generally, these limitations are accepted, or circumvented with a series of electrodes together with a feedback control loop that maintains tight control over the position of the moving parts. These limitations include the inherent stiffness of such materials, the limited linear elastic range of such materials and the complexity, hence expense of the precision lithography associated with machining such materials.
Thus, traditional silicon micromachining techniques have not provided a cost effective solution for tunable optical components for use DWDM networks.
In view of the above limitations, the present invention provides a cost-effective approach for tunable optical components with an enhanced range of motion.
This is accomplished with an optical element that is supported by a compliant member. Tunability is afforded by creating an electrostatic force that deforms the compliant member. When the force is removed, the energy stored in the compliant member restores the optical element to its initial position.
In accordance with the present invention, the compliant member is formed of an entropic, rather than an enthalpic material, with a variety of geometries including compression, tension, shear and combinations thereof. Entropic materials afford four key advantages over enthalpic materials (e.g. silicon, metals, glasses), pertaining to device response and positional/angular stability.
(1) Entropic materials (e.g. long chain homopolymers, block copolymers, elastomers, aerogels etc.) exhibit an entropic plateau region (characterized by an elastic modulus that is ca. 5 MPa or less, and is independent of frequency and strain level over a wide range of frequencies and strain levels. Enthalpic materials have an elastic modulus that is ca. 1 GPa or more, and is independent of frequency only for very small strain levels. Hence, entropic materials are far more compliant.
(2) Entropic materials have a much higher elastic limit (more than ca. 100% strain vs. less than ca. 1% strain for enthalpic materials) and thus avoid plastic deformation during actuation. This greatly enhances the achievable tuning range.
(3) Entropic materials are incompressiblexcx9cthe energy cost for volume deformation is nearly infinite, when compared to the energy cost for linear and shear deformation. This compares with enthalpic materials wherein the energy cost for volume and linear deformations are comparable. This large difference in the energy cost of deformation possessed by entropic materials can be exploited to great advantage in the design of tunable optical components. For example, when the angular misalignment of a movable optical component requires volume deformation in the compliant member, while tuning requires a shear or linear deformation only, then the energy cost for angular misalignment can become much higher than the energy cost for tuning with an entropic compliant member material, thus the device can be intrinsically more resistant to misalignment during tuning that an equivalent design that uses enthalpic materials.
(4) Entropic materials have a normal stress behavior: when they are shear deformed, they exert a so called normal stress perpendicular to the direction of shearing, in addition to the shear stress directly resulting from the shear strain. This behavior can be used to further enhance stability with specific compliant member geometries. On the other hand, enthalpic materials display a negligible normal stress behavior, and thus the normal stress behavior cannot be exploited for enhanced stabilities with enthalpic restoring layer materials.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: